Signal sectioning for profiling printed-circuit-bord vias with vertical scanning interferometry

ABSTRACT

The rough bottom surface of a recessed feature partially obscured by an overlying structure is profiled interferometrically with acceptable precision using an objective with sufficiently large numerical aperture to illuminate the bottom under the obscuring structure. The light scattering produced by the roughness of the surface causes diffused light to return to the objective and yield reliable data fringes. Under such appropriate numerical-aperture and surface roughness conditions, the bottom surface of such recessed features can be profiled correctly simply by segmenting the correlograms produced by the scan and processing all fringes that correspond to the bottom surface elevation.

RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. Ser. No.13/719,101, filed Dec. 18, 2012.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to the general field of optical metrology. Inparticular, the invention relates to a method for profiling the bottomsurface of vias in printed circuit boards when the view is partiallyobscured by preferentially oriented reinforcement fibers added to thestructure of the boards.

2. Description of the Prior Art

Printed circuit boards (PCBs) are well known structures used in theelectronic industry to mount electronic components and connect them toexternal devices. Printed circuit boards have traditionally beenmanufactured from fiber layers surrounded by a plastic matrix material.The boards have one or more layers of metalized patterns that, whenassembled with the electronic components, form electricalinterconnections among them.

In use, assembled PCBs are normally attached to a chassis, such as theframe of a computer, and are therefore subjected to stresses due tovibrations and to forces exerted by the weight of the componentsattached to them. These forces tend to produce undesirable flexing ofthe circuit boards with attendant potential loosening of the electricalconnections and separation of the electronic components. Therefore, inorder to minimize flexing, it has become common practice to reinforcePCBs by means of support structures such as reinforcing bars, beams andrib stiffeners. However, such support structures are often undesirablebecause they occupy valuable circuit-board surface area, which iscontrary to the trend of increasing the density of electrical componentson PCBs. Moreover, electrical components are becoming increasinglyheavy, thus placing an increased burden on the structure of the PCB.

In addition, ever increasing miniaturization requirements tend to leadto thinner and thinner PCBs, which therefore are also more flexible andmore subject to potential damage. U.S. Publication No. 2004/0131824provides a solution to this problem by reinforcing and stiffening thestructure of printed circuit boards in selected locations usingpreferentially oriented fibers. Selected fibers are removed from thepolymeric material matrix of the PCB and replaced with a similarquantity of different-material fibers placed in a predeterminedorientation as required to achieve the desired PCB stiffening. Becauseprinted circuit boards tend to flex along a particular axis, thereinforcing fibers are oriented transversely to resist flexure, therebyreducing material fatigue, fracture and failure. FIG. 1 illustrates asection 10 of PCB polymeric matrix where such reinforcing fibers 12 areshown laid cross the structure of the PCB. This reinforcement approachhas become common practice in the industry.

During the process of assembly of electronic components to the PCB,holes 14 (referred to as “vias” in the industry) are typically drilledwith lasers into the PCB matrix, as shown in FIG. 2, for receiving andsoldering the leads or pins of chips and other components. The vias aremetalized to form an electrical connection between the electricalcomponent pins inserted into them and the circuit board. Therefore, thevias have to be large and uniform enough to make a good and strongconnection between the printed circuit and the electronic componentswhen the vias are filled with soldering material. To that end, knowledgeof the exact dimensions of the vias is a critical part of the packagingprocess and the top and bottom diameters of the vias are measuredroutinely for quality control purposes.

Various optical systems and techniques are known that could be used toprofile vias. These include, without limitation, low-coherenceinterferometry, confocal microscopy, bright-field and dark-fieldmicroscopy (image sharpness techniques), and structured lighttechniques. These methods are all encompassed by what is generallyreferred to in the art, interchangeably, as optical metrology, opticalprofilometry, or 3-D microscopy. The signal captured in low-coherenceinterferometry (including structured light metrology) is fringes, whilein confocal microscopy, bright-field microscopy and dark-fieldmicroscopy the optical signal is irradiance.

When the dimensions of the via are measured with low-coherenceinterferometric (WLI) techniques, the via is scanned with aninterferometric objective with a field of view exceeding the topaperture of the via (that is, an objective overlying the entire viaopening or a system adapted to cover that area through stitching of dataacquired with high numerical-aperture objective and a smaller field ofview) and the top and bottom surfaces are profiled by identifying inconventional manner the scanning heights where local fringe modulationmaxima are produced during the scan through focus. However, it has beenfound that drilling vias in PCBs reinforced with oriented fibers doesnot produce uniform tubular structures because the reinforcing fibermaterial tends to melt away at a different rate than the PCB matrix whendrilled with a laser and leave behind loose fibers that form asubstantially annular shelf 16 that protrudes into the vias 14 (FIG. 2).As a result, the shelf 16 is an impediment to the WLI measurement of theportion of the bottom surface 18 of the via under the shelf because itsview is obscured to the overlying scanning objective 20, as illustratedin FIG. 3.

The conventional approach has been to scan through the height of the viaand obtain its dimensions by identifying the position of maximum fringecontrast for each pixel by some method, such as the center of gravityapproach. That is, the detector pixels recording light irradiancereceived from the top surface of the PCB will produce a correlogram withlocal maximum contrast at corresponding scan positions; the pixelsrecording irradiance received from the bottom surface of the via visiblefrom the top will produce another correlogram with a local maximumcontrast at scan positions corresponding to the bottom; and, similarly,the surface of the fibers constituting the intermediate annular shelfproduces a correlogram characterized by well-defined fringes at thescanning positions corresponding to the shelf height. Because theportion of the via's bottom surface below the fiber shelf is obscured tothe interferometer's objective by the overlying shelf, no fringes havebeen expected to be produced by the bottom regions under the shelf.Therefore, any modulation detected by detector pixels corresponding tothese bottom regions of the via has been considered noise anddisregarded or treated as insignificant by the algorithms used toprofile the bottom of the vias. As a result, the geometry of the bottomsurface of vias has been measured based only on the information obtainedfrom detector pixels corresponding to the visible portion of the surface(that is, the portion that is not obscured by the fiber shelf).

It is clear that the conventional approach leads to incorrectmeasurements because it is known that the bottom of vias is larger thanthe visible portion inside the inner perimeter of the fiber shelf aboveit. The present invention provides a simple solution to this problem.

SUMMARY OF THE INVENTION

The invention is based on the discovery that under certain particularconditions the bottom surface of a PCB via can be profiled withacceptable precision using WLI even when its view is obscured by thepresence of fibers protruding into the via opening. This is contrary toexpectation because scanning interferometry is based on interferencebetween a reference beam and a sample beam reflected from the surfacebeing measured. Therefore, a clear view of the target surface from theobjective of the scanning interferometer is always considered essentialto a good measurement. However, it was found that the bottom of the viacan be measured adequately if the conditions are such that sufficientlight reaches the bottom surface below the obstructing fibers and isdiffusively reflected back to the scanner's objective.

As a first criterion for the invention to work, the numerical apertureof the objective must be sufficiently large to illuminate the portion ofthe surface under the fibers at the bottom of the via and to allow thereflected light to return to the objective to produce measurementfringes by interfering with the reference beam. Given the typicalplacement of reinforcing fibers within the matrix of the printed circuitboard and the normal extent of their protrusion into the via opening, itwas found that a numerical aperture of at least 0.4 in a 20×interferometric objective often provides the necessary angle for thesample beam to illuminate the bottom surface of the via with sufficientreflected light to return to the objective to produce an acceptablemeasurement.

The roughness of the surface at the bottom of the via is anothercritical and contributing factor to the effectiveness of the invention.Because surface roughness produces light scattering that in turn resultsin an effective increase of the ability of reflected light to return tothe objective past the shelf of reinforcing fibers, it is important thatthe bottom surface of the via be sufficiently rough to scatter thereflected light up past the fibers toward the objective. Fortunately,the vias produced by laser drilling tend to have a roughness in theorder of a fraction of a micron (typically 300-700 nm), so this factoris inherently present in the applications for which the invention isgenerally intended.

Under such appropriate numerical aperture of the objective and surfaceroughness conditions, we discovered that sufficient sample-beam lightreaches the area of the bottom surface of the via under the fiber shelfand is diffused back to the objective to enable the measurement of theentire bottom surface simply by processing in its entirety thecorrelogram corresponding to the bottom surface elevation. That is,rather than discarding as noise the fringes produced at pixels under theshelf of protruding fibers in the via, those fringes are processed astrue surface data. Contrary to expectation, we found that surfacemeasurements so conducted lead to materially more precise profiles thanpreviously possible.

Various other features and advantages will become clear from thedescription of the invention in the specification that follows and fromthe appended claims. Therefore, to the accomplishment of the objectivesdescribed above, this invention consists of the features hereinafterillustrated in the drawings, fully described in the detailed descriptionof the preferred embodiments, and particularly pointed out in theclaims. However, such drawings and descriptions disclose only some ofthe various ways in which the invention may be practiced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational schematic representation of a section ofprinted circuit board reinforced with a layer of oriented fibers.

FIG. 2 illustrates vias drilled in the PCB of FIG. 1 and the shelf offiber residue remaining in each via as a result of a laser drillingoperation.

FIG. 3 illustrates the obstruction to WLI illumination provided by theshelf of reinforcing fibers formed by drilling in each via.

FIG. 4 illustrates four correlograms recorded at four pixelscorresponding to the top surface, the intermediate fiber shelf, and thecentral area of unobscured bottom of a typical PCB via.

FIG. 5 illustrates the result of a conventional measurement of a PCB viawhere the approximately circular bottom area is measured to correspondto the inner perimeter of the fiber shelf produced by the laser drillingprocess.

FIG. 6 illustrates the result of a measurement according to theinvention of the PCB via of FIG. 5 where the approximately circularbottom area is measured to exceed the inner perimeter of the fiber shelfproduced by the laser drilling process and to substantially match theactual size measured by other means.

FIG. 7 illustrates the effect of surface roughness on the light returnedfrom the bottom surface of a via for interference with theinterferometer's reference beam.

FIG. 8 is a flow-chart of the basic steps of the process of theinvention.

FIG. 9 illustrates a generic recessed feature where the bottom surfaceis partially obscured by an overlying layer of material and the topsurface of this material is at least in part obscured by intrusions fromthe top of the feature.

FIG. 10 illustrates a generic recessed feature where the bottom surfaceof the feature is partially obscured by intrusions from the top of thefeature.

FIG. 11 illustrates a generic open surface partially obscured by anoverlying shelf-like structure.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “shelf” refers to the substantially annularstructure of reinforcing fibers produced by the process of forming a viain a printed circuit board. As a result of the different materialsconstituting the reinforcing fibers and the rest of the PCB, the bottomsurface of the via includes a correspondingly annular area that isobscured by the overlying fiber shelf and not directly visible from thetop opening of the via. The terms “diffused” and “scattered” and theirderivatives are used synonymously.

The invention is described below with reference to low-coherenceinterferometry (normally referred to in the art also as low-coherencewhite light interferometry—WLI—or vertical scanning interferometry—VSI).However, it is recognized that it is applicable to any through-focusoptical method of measurement. Therefore, for the purposes of claimingthe invention, the term “through-focus” is intended to encompass anyoptical approach whereby an optical signal is captured while a samplesurface is passed through the focal point of an objective, such aslow-coherence interferometry, confocal, bright-field and dark-fieldmicroscopy. In the case of low-coherence interferometry, the opticalsignal is fringes that yield a process signal referred to as modulation.In the case of confocal microscopy, the optical signal is irradiancethat is typically processed as such. In the case of bright-field anddark-field microscopy, the optical signal is irradiance that is normallyprocessed in terms of its standard deviation within neighboring pixels.

FIG. 4 illustrates four correlograms recorded for four pixelscorresponding to different areas in the field of view of aninterferometric objective scanning a typical PCB via. The firstcorrelogram on the left shows fringes only at the top of the via,thereby contributing to defining the top contour of the opening withinthe surface of the PCB. As expected, no light goes below the solidsurface and no other fringes are produced during the scan. The secondand third correlograms correspond, respectively, to a pixel near the viawall overlooking the fiber shelf produced by drilling of the via and toanother pixel also over the shelf, but more centrally located away fromthe via wall. As expected, the pixel over the shelf near the wall(second correlogram) shows a single set of fringes at the height of theshelf because that is the only elevation from which light can bereflected. The third correlogram, on the other hand, shows two sets oflocal fringes; one at the height of the shelf and another, much smaller,set of fringes below the shelf and near the bottom of the via. Thissmaller set of local fringes has been discarded in the prior art asnoise. The fourth correlogram, on the right of FIG. 4, corresponds to apixel over the unobstructed bottom of the via. Also as expected, itshows a corresponding single set of fringes at the bottom of the via.

As mentioned, prior-art interferometric procedures routinely discard thebottom set of local fringes (seen in the third correlogram) as noisydata because corresponding to a surface obscured by the fiber shelfoverlying that portion of the bottom surface. As such, algorithmstypically reject it using irradiance thresholds designed to isolate andeliminate noise. As a result, the interferometric profile of the viaproduces an approximately circular top opening, an annular shelf at somedepth in the via, and a similarly approximately circular bottom surfacecorresponding to the inner perimeter of the shelf (i.e., the areadirectly exposed to the field of view of the scanning objective).Elliptical curves are typically fitted to the data and their major andminor axes are used to define the various shapes for quality-controlpurposes (for example, the average between the major and minor axes isused as an effective, average diameter of an approximately circularstructure).

A typical PCB via is about 20-60 microns deep and has an averagediameter of about 40-60 microns. (Average diameter is often used becausevias are not perfectly cylindrical.) The fiber shelf created duringdrilling is about 2-10 microns wide at a depth corresponding to thelayer of preferentially oriented reinforcement fibers. FIG. 5illustrates the conventional measurement result obtained scanning such avia. The measurement produced a via depth of approximately 41 microns,with a top opening 30 with an average diameter of about 60 microns, afiber shelf 32 about 9-micron wide (i.e., with an averageinner-perimeter diameter of about 42 microns) placed about 20 micronsfrom the top, and a correspondingly equal circular bottom area 34 ofabout 42 microns in average diameter. These results illustrate the factthat this conventional approach to measuring vias interferometricallydoes not produce a reliable measurement below the shelf 32 because itdoes not show that the surface of the bottom area 34 extends below theshelf even though that is known to be the case.

The essence of the present invention lies in the fortuitous discoverythat the data provided by the fringes heretofore considered noise (thebottom fringes of the third correlogram in FIG. 4) can be usedadvantageously to improve the measurement of the bottom surface of vias.If such data are combined with the correlograms corresponding to thecentral part of the bottom surface (represented by the fourthcorrelogram in FIG. 4), the scan of the bottom surface of the viaproduces a measurement that has been found to represent the actualgeometry of the via materially more accurately than believed possiblewith vertical scanning interferometry. To that end, according to theinvention the via is normally scanned from top to bottom (or vice versa)as previously done, but the data are segmented vertically as pertainingto the top region, the shelf region, and the bottom region. (Forclarity, the term “segmenting” is used to indicate the separation ofdata pertaining to different heights obtained from an opticalthrough-focus measurement of a structure obscuring a surface under thestructure.) The top and shelf regions are profiled as usual, using thefringe data generated by the respective structures during the scan attheir respective heights. However, all of the data generated by thebottom structure are used to calculate a profile of the bottom area,including the fringes previously discarded as artifact or noise. FIG. 6illustrates the results obtained from the interferometric data of FIG. 5using the segmented approach of the invention. While the area of theopening 30 and the shelf 32 (not shown) remained the same, the surfaceof the bottom area 34 is measured with an average diameter of about 47microns, showing that it extends past the 42-micron inner perimeter ofthe shelf, as expected.

It is believed that this measurement is made possible by light that isscattered off the bottom area 34 from roughness that directs it back tothe objective even from incidence locations that do not seem to beaccessible to the objective, such that sufficient light is collected,even from below the shelf, to provide meaningful fringe data during thescan. FIG. 7 illustrates such effect on light scattered by roughness inthe bottom surface of the via. As mentioned, the vias drilled with alaser beam necessarily have a surface with a degree of roughnesssufficient to provide the necessary scatter to reflect light from underthe shelf.

Clearly, however, for the invention to work it is above all necessarythat some light reach the bottom in areas lying under the shelf. Thisillumination is achieved by utilizing an objective with a numericalaperture (NA) sufficiently high to guarantee that enough light isprojected at an angle below the shelf and diffused back toward theobjective to produce measurable fringes. As would be clear to oneskilled in the art, the minimum NA suitable for a particular measurementwill depend on the exact location of the shelf within the height of thevia (and the width of the shelf), but such minimum NA can be easilyascertained empirically or estimated by theoretical calculations. Forexample, for the via illustrated by the measurements of FIGS. 5 and 6,we found that a NA of at least 0.4 in a 20× interferometric objectivewas necessary to obtain consistent bottom area results.

Because the measurements of vias for quality-control purposes areperformed repeatedly on printed circuit boards having essentially thesame structure (with corresponding vias that ideally are also the sameand have comparable bottom-surface roughness), the invention is bestcarried out by first measuring a via using objectives with increasingnumerical apertures until the measurement of the bottom area of thatparticular type of via remains substantially unchanged with increasingNAs greater than a minimum value. That would indicate that in all casesa sufficient amount of light had reached the hidden bottom surface toproduce reliable results given the roughness of the via bottom; thus,any of those NAs could then be used with confidence for subsequentmeasurements of that type of via.

Given the fact that PCB vias are inherently characterized by asufficiently rough bottom surface (in the order of 300-700 nanometersRa) following this approach guarantees that the measurements obtainedusing segmented fringe data according to the invention are reliable andrepeatable. In fact, the same via used for the measurements reported inFIGS. 5 and 6 was sectioned mechanically below the shelf and the bottomsurface was measured optically under completely open conditions. Themeasurement produced an average diameter in close agreement with themeasurement produced by the segmented approach of the invention (alwayswithin 2-3 microns).

Thus, a simple but effective new WLI approach has been disclosed toprofile the bottom area of PCB vias formed by drilling the board througha layer of oriented reinforcing fibers. The results have shown to becorrect and repeatable so long as a sufficiently large numericalaperture is used in the scanning objective. FIG. 8 is a flow-chart ofthe basic steps of the process of the invention.

It will be clear to one skilled in the art that the invention may alsobe practiced by measuring only the bottom of the via; that is, withoutscanning through the entire height of the via. The invention may also beused to measure any recessed feature having a bottom surface that ispartially obscured by an intermediate layer of material placed betweenthe top and the bottom of the feature. Similarly, any surface below andobscured by an overlying shelf-like layer of material, even if not thebottom surface of a recess, can be measured according to the inventionif the surface has the necessary degree of roughness to diffuse lightback toward the objective. The same elements of the invention apply aswell to all of these conditions of scattered illumination. As shown inFIG. 9, for example, if the top surface 36 of a shelf 38 is covered byoverlying structures 40 and has the required roughness to produce adiffusive reflection to be collected by an objective 20 with theappropriate numerical aperture, the surface 36 (as well as the obscuredportions of the bottom surface 42) can be measured with the technique ofthe invention in spite of the obstructions over them. The correlogram Cillustrates the fringes produced at the various depths of a scan atpixels corresponding to locations over the structures 40 and shelf 38,as shown in the figure. Given the same conditions, the same techniquewould make it possible to improve the measurement of the bottom surface42 of a feature partially obscured by structures 40 at its top, as shownin FIG. 10 (that is, a feature with no intermediate shelf). FIG. 11illustrates an open surface 42 partially obscured by a shelf-likestructure 40, for which the same principles of the invention apply.

Various changes in the details that have been described may be made bythose skilled in the art within the principles and scope of theinvention herein illustrated and defined in the appended claims. Forexample, the invention has been described in terms of vias drilled witha laser in the printed circuit board, but it is understood that it couldbe applied to any via formed by whatever means through an intermediatelayer of reinforcing material that partially obscured the bottom of thevia. The recessed feature does not have to be round, but it could haveany geometry compatible with an intermediate layer of material partiallyobscuring the bottom of the feature. It could be a longitudinal trenchwhere the bottom has been formed by undercutting the top layer. Also,any through-focus based method (such as interferometric, confocal,bright-field, and dark-field) using a high numerical aperture objectivefor shape measurement is expected to benefit from the invention becausethe same conditions of high NA and bottom surface roughness would causethe light to reach under the shelf and be partially scattered back tothe objective, thereby providing information about the surface under theshelf. Thus, while the invention has been shown and described in whatare believed to be the most practical and preferred embodiments, it isrecognized that departures can be made therefrom within the scope of theinvention, which is not to be limited to the details disclosed hereinbut is to be accorded the full scope of the claims so as to embrace anyand all equivalent apparatus and methods.

What is claimed is:
 1. An optical method of measuring a surface of arecessed feature containing an overlying layer of material obscuring atleast a portion of said surface of the feature, the method comprisingthe following steps: performing an optical through-focus measurement ofthe recessed feature to produce surface information corresponding tosaid surface of the feature and layer information corresponding to theoverlying layer and to a portion of a surface of the feature obscured bythe overlying layer; segmenting said layer information to isolate recessinformation corresponding to said portion of the surface of the featureobscured by the overlying layer; and profiling the surface using saidrecess information corresponding to the portion of the surface of thefeature obscured by the overlying layer in the feature; wherein saidperforming step is carried out with an objective having a numericalaperture and with said surface of the feature having a roughness so thatan illuminating beam produces a diffusive reflection from said portionof the surface of the feature obscured by the overlying layer and saiddiffusive reflection is directed back to the objective.
 2. The method ofclaim 1, wherein said performing step includes identifying a numericalaperture of the objective such that said portion of the surface of thefeature obscured by the overlaying layer of material is illuminated bysaid illuminating beam.
 3. The method of claim 2, wherein said recessedfeature is a via in a printed circuit board, and said overlying layer ofmaterial obscuring a portion of the surface of the via is a fiber shelfresulting from laser drilling the via in a printed circuit board.
 4. Themethod of claim 3, wherein said numerical aperture is at least 0.4 in a20× interferometric objective.
 5. The method of claim 4, wherein saidsurface of the via has a roughness of at least 300 nanometers Ra.
 6. Themethod of claim 3, further including the step of calculating an averagediameter of the via.